Increased sympathetic nervous activity is associated with enhanced release, turnover and metabolism of norepinephrine. In order to maintain the function of this system, norepinephrine synthesis must increase. There are apparently three ways by which this increase occurs: (1) Reduced end-product feedback inhibition; (2) an immediate post-stimulation increase in norepinephrine synthesis of unknown mechanism; and (3) increased enzyme synthesis with more prolonged sympathetic stimulation. The mechanisms by which norepinephrine synthesis is regulated will be examined in more detail, primarily in the isolated hypogastric nerve-vas deferens preparation of the guinea-pig. The role which free intraneuronal norepinephrine, other norepinephrine stores, nerve terminal depolarization and stimulus-secretion coupling play in enhancing norepinephrine synthesis, both during stimulation and in the post-stimulation peroid, will be evaluated. A comparison of changes in tyrosine hydroxylase levels and tyrosine hydroxylase activity in situ will be made in control tissues and after a variety of drug and environmental stresses of different durations. The role of ribosomal and mitochondrial protein synthesis, axoplasmic transport, cyclic AMP and prostaglandins on the regulation of norepinephrine synthesis will be evaluated. The role of sex and steroid hormones in the responses of the vas deferens to nerve stimulaton and the regulation of norepinephrine synthesis will be studied. The rate of equilibration of tyrosine in the adrenergic neuron will be considered. Attempts will be made to determine the importance of newley synthesized norepinephrine as a contributor to the pool of norepinephrine released by nerve stimulation. Studies of tyrosine hydroxylase induction will be made using mouse neuroblastoma cells in culture as a model system. Tyrosine hydroxylase will be further characterized with regard to substrate and inhibitor kinetics and the role of pteridines and pteridine reductase in regulation of norepinephrine synthesis will be examined. Analogous studies on tryptophan hydroxylase will be initiated.